Electrolytic coloring of anodized aluminum

ABSTRACT

THIS INVENTION IS ADDRESSED TO A PROCESS FOR ELECTROCHEMICALLY COLORING ANODIZED ARTICLES OF ALUMINUM OR ALLOYS OF ALUMINUM BY ELECTROLYTIC DEPOSITON OF HEAVY METALS OR HEAVY METAL COMPOUNDS, COMPRISING MOUNTING THE ARTICLE IN A BATH OF AN AQUEOUS SOLUTION OF A GIVEN METAL SULFAMATE AND PASSING A DIRECT CURRENT THROUGH THE BATH.

United States Patent ()flice Patented Jan. 29, 1974 3,788,956 ELECTROLYTIC COLORING F ANODIZED ALUMINUM Jos Patrie, Grenoble, Jacques Lefebvre, Voiron, Roger Segond, Saint-Cloud, and Michel Badia, Echirolles, France, assignors to Cegedur Societe de Transformation de lAluminum Pechiney, Paris, France No Drawing. Filed June 23, 1972, Ser. No. 265,653 Claims priority, application France, June 25, 1971, 7123388; Sept. 10, 1971, 7132743 Int. Cl. C23b 9/02 U.S. Cl. 204-35 N 9 Claims ABSTRACT OF THE DISCLOSURE This invention is addressed to a process for electrochemically coloring anodized articles of aluminum or alloys of aluminum by electrolytic deposition of heavy metals or heavy metal compounds, comprising mounting the article in a bath of an aqueous solution of a given metal sulfamate and passing a direct current through the bath.

This invention relates to a process for electrochemically coloring preanodized articles of aluminum or aluminum alloys by the electrolytic deposition of heavy metals or heavy metal compounds into the pores of the oxide layer.

In this specification, the term direct current covers any form of one-way current, such as rectified and non-filtered currents and currents that are even periodically interrupted, as in the case of the rectification of a single alternation of a single-phase current.

It is known that an article of anodized aluminum can be colored by causing particles of metal or metal compounds to penetrate into the pores of the layer of alumina through electrolysis in an aqueous acid solution of a heavy metal salt, such as Ni, Co, Cu, Ag, Pb, Sn. Heretofore, electrolysis has generally been carried out with sinusoidal alternating current voltage.

However, it is also known from our French Pat. No. 2,052,100 that a direct current can be superimposed upon the alternating current, although inthis 'case the peak voltage of the alternating current has to be distinctly higher than the AC. voltage superimposed on it so that a reversal in the direction of the electrolysis current is obtained in each cycle.

It has also been proposed to carry out coloring by direct-current electrolysis in a bath of heavy metal salts after having subjected the metal, anodized with direct current in the usual way, to alternating-current electrolysis for a few minutes in the anodizing bath (published German patent application 1,948,552). According to this application, it is essential to carry out this intermediate treatment with alternating current before the cathodic treatment in the bath containing the heavy metal salts; otherwise the oxide film is destroyed or is not colored or is irregularly colored.

According to the prior art, the use of alternating current is thus absolutely essential if uniform color finishes are to be obtained by electrolysis in a solution of metal salts.

Unfortunately, the use of alternating current involves difliculties. Although it is possible with a nickel sulfate bath to obtain light bronze or dark bronze color finishes, it is not possible to obtain a black color finish. Penetration is mediocre and the electrical conditions have to be closely controlled to prevent detachment of the oxide layer. One serious disadvantage of this process is the difficulty of obtaining reproducible color finishes. In addition, it has been found that the surface ratio of the electrodes has an important bearing on coloring, above all in the case of nickel baths. Accordingly, allowance has to be made when installing the electrodes in the cell for a certain ratio between the surfaces of the electrode and the counter-electrode.

Although this process enables disruption of the layer to be avoided, it is not sufficient to guarantee reproducibility.

In addition, the layer of alumina acts as an imperfect rectifier for the electrical current, with the result that the momentary intensity of the current is not merely a function of the momentary voltage. It is governed by the previous anodization and fluctuates during the coloring electrolysis operation. The amount of electricity which travels in the direction required for deposition of the metal into the layer of alumina cannot readily be determined. It is for this reason that the production of a uniform color finish which is identical throughout a series of articles calls for extremely strict control of all the operations involved in preliminary anodization and coloring electrolysis.

It is accordingly an object of the present invention to provide a method for coloring surfaces of anodized aluminum or alloys of aluminum which overcomes the foregoing disadvantages of the prior art, and it is a more specific object of the invention to provide a method for coloring anodized aluminum or aluminum alloy articles with direct current.

The concepts of the present invention reside in a method for coloring anodized articles of aluminum or an alloy of aluminum in which the article is mounted as the cathode in an electrolysis bath of an aqueous solution of a heavy metal sulfamate and a direct current is passed through the bath.

In this way, the process can be applied with the electrical equipment available in a conventional anodizing plant. No difficulties are encountered in accurately measuring the amount of electricity passing through the bath. Since the penetrating power of the metal is extremely high, the surface ratio of the electrodes has hardly any bearing on the process, and satisfactory reproducibility is obtained with much more ease than in conventional baths without any need for agents intended to improve the penetrating power to be added to the bath.

In order to fix the pH-value at the required level, which is governed by the type of metal used, free sulfamic acid is added as and when required, and a buffer, preferably boric acid, is used to keep the pH-value constant.

The pH-value should be between 1 and 1.5 for the cations silver, lead, tin, which necessitates the addition of sulfamic acid. Nickel requires a higher pH-value amounting to between 3.5 and 5.5, so that there is generally no need to add acid.

The direct-current electrolysis in a sulfamic bath gives an excellent Faraday efiiciency so that the quantity of metal deposited and the coloring of the article are influenced hardly at all by the working conditions. The concentration of heavy metal sulfamate can vary within very wide limits without causing appreciable differences in the color finish obtained. The type and size of the anode have hardly any bearing at all and the current density can be selected within a very wide range of from 0.05 to 3 amps/dm. without involving any modifications other than the duration of the operation.

In practice, the nickel sulfa-mate is used in concentrations of from 0.2 to 1.6 mole/ liter with a nickel counterelectrode with a surface from 0.1 to two times the surface of the article to be colored which has the advantage that it is possible successively to treat aluminum articles differing widely from one another in their size in the same bath without having to modify the anode surface.

In the case of silver baths, it is preferred for obvious reasons of economy to use dilute solutions of silver sulfamate containing from 0.002 to 0.01 mole/liter with a graphite counter-electrode. In the case of lead or tin,

extremely good results have been obtained with concentrations of from 0.01 to 0.1 mole per liter. In this case, the counter-electrodes can be formed of metal, namely lead or tin, or of graphite.

The preferred current density is that which corresponds to a 2 to -minute duration of the operation which simultaneously provides for fast coloring and strict control of the time, resulting in extremely good reproducibility.

The electrolysis operation is carried out at temperatures in the vicinity of ambient temperature, and preferably at temperatures of from to C.

In order to obtain deep bronze and black color finishes with a solution of nickel sulfamate, it is of advantage to apply the direct current intermittently, the article to be treated being discharged during the breaks in the current.

In the present context, the term discharge is used in a broad sense because the phenomenon of discharge or depolarization of the article to be treated is not exactly known. It would appear that it is similar to that which is observed in the case of an electrolytic capacitor. During the breaks in the current, there occurs an inverse flow of current whose momentary initial intensity can be extremely high, although it decreases very rapidly and finally disappears altogether. Discharge of the article to be treated makes the subsequent active period much more effective than it would be had the break in current not taken place.

At a constant current density, the voltage observed in each cycle is lower than that which would have been found in continuous operation at the corresponding instant. At constant voltage, the total quantity of electricity is greater than that which it would be possible to pass through in continuous operation.

The article to be treated is advantageously discharged by short-circuiting with the anode.

A switching system can be used for interrupting the direct electrolysis current and for short-circuiting the electrodes with a predetermined frequency and for a predetermined time. The duration of these cycles can either be constant or variable in time.

The direct current is preferably applied for a time t of from about two seconds to three minutes, the period for which the current is interrupted being between t and approximately /5 t.

The choice and the duration of the cycles and of the cyclic ratio is determined experimentally by observing the voltage or current intensity curves as a function of time.

The operation can be carried out either with a constant D.C. voltage source or with a source of constant-intensity current.

In cases where a constant voltage source is used, it has been found that the density of the current decreases as a function of time during the instant for which the electrodes are connected to the source. After the period during which the electrodes are stopped and short circuited, the current density is substanttially restored to the value which it had at the beginning of the preceding cycle, and then decreases up to another interruption. The same phenol nenon is repeated in each cycle.

In cases where a constant-output source is used, it has been found that the voltage at the terminals of the cell increases as a function of time during the instant for which the electrodes are connected to the source. After the period during which the electrodes are stopped and short-circuited, the voltage at the terminals is substantially restored to the value which is had at the beginning of the preceding cycle and then increases up to another interruption.

In practice, however, it is preferred, after each break in the current, to restore the voltage by allowing it to increase progressively from 0 to its regulation value over a period of from to /3 I.

7 By applying periodically interrupted direct current, it is possible for example to pass a quantity of current greater than that which would have been observed in continuous operation over a given effective period under constant voltage. For example, an anodized profile of Al-Mg-Si treated in a nickel sulfamate bath under a D.C. voltage of 14 volts for a period of 210 seconds without interruption, becomes very deep bronze in color, although it is impossible to exceed this color without destroying the layer of alumina. The color finish obtained by working in cycles for the same effective time is deep black.

Having described the basic concepts of the invention, reference is now made to the following examples, which are provided by way of illustration and not by way of limitation, of the practice of the invention.

EXAMPLE 1 Nickel bath: An electrolysis bath is prepared by dissolving in water:

220 g./l. of nickel sulfamatc (Ni(SO NH -4H O and 27 g./l. of boric acid.

This bath has a pH-value of 4.5 and its temperature is maintained between 20 and 25 C. It is used for coloring sheets of commercial-grade 99.5% aluminum, previously anodized to 18 microns in a sulfuric bath and with direct current in accordance with the conventional process. These plates are used as cathode while the anode consists of a nickel plate with the same surface area as the aluminum plates.

(a) Electrolysis tests carried out at different voltages for a constant period of 180 seconds:

9 volt--color obtained: light bronze ll volt-color obtained: medium bronze 13 voltcolor obtained: deep bronze (b) Electrolysis tests carried out over different periods of time with a constant current density of 0.1 amps/dm.

1 minute-color obtained: light bronze 3 minutescolor obtained: medium bronze 5 minutescolor obtained: deep bronze.

EXAMPLE 2 The composition of the bath is as follows:

G./l. Nickel sulfarnate (hydrated) Boric acid 50 The bath has a pH-value of 4.3 and is kept at a temperature of 35 C.

The test is carried out with the same plates as in Example 1 with the same nickel anode, the current densities and the durations being varied.

0.1 amps/dmfi, for 1 minute: light bronze 0.15 amps/dmfi, for 2 minutes: medium bronze 0.15 amps/dm. for 5 minutes: deep bronze 0.2 amps/dmF, for 8 minutes: black EXAMPLE 3 With the same bath as in the preceding example, an aluminum plate anodized to 15 microns in a sulfamic acid bath is used as the cathode. To color the plate, a current density of 0.6 amps/dm. is passed for two minutes which requires a voltage of 28 volts at the terminals of the cell. An anthracite-gray color finish is obtained, which is distinctly different from the bronze color finishes obtained on plates preanodized in a sulfuric bath.

Examples 1 to 3 show that the intensity of the color finish is directly related to the quantity of electric current passing through the cell. This enables the electrical conditions to be automatically regulated to obtain a predetermined color finish. Accordingly, the process is much easier to work on an industrial scale than electrolysis processes using alternating current.

EXAMPLE 4 A silver bath with the following composition is used:

G./l. Silver sulfamate 1.5

Boric acid 30.0 Sulfamic acid in a quantity sufiicient to adjust the pH-value to 1.

The test is carried out with the same aluminum panels as in Example 1 and with a graphite anode.

A golden yellow color finish is obtained after 1 minute at 1.6 amps/dm. while a deep yellow color finish is obtained after 2 minutes at 1:6 amps/dmP.

EXAMPLE 5 A lead bath with the following composition is used:

G./l. Lead sulfamate 1.5 Boric acid 30.0 Sulfamic acid in a quantity sufficient to adjust the pH-value to 1.3.

The test is carried out with the same aluminum panels as in Example 1 and with a lead anode of the same surface area as the aluminum panels.

A light bronze color finish is obtained after 2 minutes at 0.4 amps./dm. while a deep bronze color finish is obtained after 5 minutes with a current density increasing linearly with time from 0.2 to 0.8 amps/dmF.

EXAMPLE 6 A tin bath with the following composition is used:

G./l. Stannous sulfamate Sulfamic acid Boric acid 30 4 EXAMPLES WITH INTERMITTENT APPLICATION OF THE DIRECT CURRENT In the following examples, an aqueous solution containing 125 g./l. of hydrated nickel sulfamate and 50 g./l. of boric acid is used as the electrolysis bath. This solution has a pH-value of 4. Electrolysis is carried out at 35 C. in a 250-liter cell (except in Example 11 where the cell has a capacity of 4,000 liters) with a counterelectrode of nickel having the same surface area as the aluminum electrode. The aluminum electrode is connected to the negative terminal of a current source whose voltage or intensity can be adjusted to a constant value after a progressive increase in voltage over a period of 10 seconds after each break in the current.

When the switch cuts off the current, an electrical connection is established between the two electrodes which are thus short-circuited. An ammeter connected in series with the short-circuit connection indicates the time at the end of which disappears the current due to the discharge.

EXAMPLE 7 After a total treatment time of 6 minutes, of which 4.5 minutes represent effective time, the aluminum article has taken on a deep black color. The maximum voltages at the terminals measured just before the current is broken are, successively, 11.3, 10.5, 9 and 8.6 volts.

EXAMPLE 8 An identical panel is treated in the same bath, but with a current source equipped with an automatic voltage regulator which 'fixes the voltage at 14 volts after a substantially linear increase from 0 to 14 volts over a period of 10 seconds. The cycle is made up of a 30 second halfcycle in which effective coloring takes place, and a 30 second half-cycle during which discharge occurs. After 8 cycles corresponding to an effective time of 4 minutes 30 seconds, out of a total period of 8 minutes 30 seconds, the article is deep black in color.

EXAMPLE 9 A profile of Al-Mg-Si alloy which has a surface area of 50 dm. and which has been preanodized in a sulfuric bath, is treated. The current source is provided with an automatic regulator which fixes the intensity at 10 amperes. Each cycle comprises 10 seconds of progressive application of voltage, 50 seconds of electrolysis at 10 amperes and 30 seconds of discharge. After 8 cycles, having lasted a total of 11 minutes 30 seconds and corresponding to an effective duration of 8 minutes, the profile has acquired a deep black color. The voltages observed at the end of each of the 8 cycles, just before the current is interrupted, are, respectively, 10.5, 10.2, 8.9, 8.1, 7.6, 7.2, 6.8 and 6.5 volts. Accordingly, the color finish is obtained at a constant current density of 0.2 amps/ drn. under considerably lower voltages than the final voltages of around 32 volts obtained in continuous operation, and it is possible to obtain a black color finish which would have been substantially impossible in continuous operation.

EXAMPLE 10 A profile identical to that used in Example 9 was treated using a constant voltage source of 14 volts. Each cycle is made up of 10 seconds of increase in voltage, 20 seconds of coloring under 14 volts and 30 seconds of discharge. Seven successive cycles were applied, given a 5 total duration of 7 minutes of which 3 minutes 30 seconds represent effective time. In each cycle, the maximum intensity is of the order of 30 amps, while the minimum intensity just before the current is broken fluctuates between 8 and 10 amperes. The profile is colored deep black.

EXAMPLE 1 l The preceding test Was repeated in an industrial 4,000 liter cell of which the nickel sulfamate bath had already been used to color more than 800 m? of aluminum articles. The test was carried out on 6 m profiles of Al- Mg-Si preanodized in a sulfuric bath. A constant voltage source equal to 14 volts was used, each cycle being made up of 10 seconds voltage application, 5.0 seconds of coloring at 14 volts and 30 seconds of discharge. 5 cycles are sufiicient to obtain a deep black color finish on the profiles.

It will be understood that various changes and modifications can be made in the details of formulation and procedure without departing from the spirit of the invention, especially as defined in the following claims.

What is claimed is:

1. A process for electrochemically coloring anodized articles of aluminum or alloys of aluminum by electrolytic deposition of heavy metals or heavy metal compounds, comprising mounting the article as the cathode in a bath of an aqueous solution of a heavy metal sulfamate and passing a direct current through the bath.

2. A process as claimed in claim 1 wherein the aqueous solution contains nickel sulfamate in a concentration of from 0.2 to 1.6 mole/liter and boric acid as a buffer in a concentration of from 25 to 50 g./liter, the pH being in the range from 3.5 to 5.5.

3. A process as claimed in claim 1 wherein the aqueous solution contains lead or tin sulfamate in a concentration of from 0.01 to 0.1 mole/ liter, boric acid as a buffer in a concentration of from 25 to 50 g./liter and sulfamic acid in a quantity suflicient to adjust the pH-value to between 1 and 1.5.

4. A process as claimed in claim 1 wherein the aqueous solution contains silver sulfamate in a concentration of from 0.002 to 0.01 mole/liter, boric acid in a concentration of from 25 to 50 g./ liter and sulfamic acid in a quantity sufficient to adjust the pH to between 1 and 1.5.

5. A process as claimed in claim 1 wherein the sulfamate is nickel sulfamate and the direct current is intermittently applied, the articles to be treated being electrically discharged during the breaks in the current.

6. A process as claimed in claim 5 wherein the article to be treated is electrically discharged by short-circuiting with the anode.

7. A process as claimed in claim 5 wherein the direct current is applied for a time t of from about 2 seconds to References Cited UNITED STATES PATENTS 3,704,2l0 11/1972 Patrie 204-58 3,634,208 1/1972 Kuroda et al 204-58 FOREIGN PATENTS 662,063 4/1963 Canada 204-58 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 788, 956 Dated January 29, 1974 Inventor-(s) Jos Patrie, et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In column 1, line 10, please change "7l23388" to Signed and sealed this 29th day of April 1975.

(SEAL) Attest:

' T C. MARSHALL DANN RUTH C. MASON Comiss-ioner of Patents Attesting Officer and Trademarks 

